How to Improvement of Dispersibility of Rubber Compound

Poor dispersion of carbon black will lead to a decrease in the physical properties of the vulcanised compound and deterioration of the product properties, such as non-abrasion resistance.

The following experimental protocol may improve the dispersion of carbon black and other fillers.The reader is encouraged to do his/her own research and reading of the relevant literature sources in the book, including those cited later.Note: These generalised experimental protocols may not be applicable to every specific case.Any variable that improves the dispersion of carbon black will certainly affect other properties, for better or for worse, but changes in other properties are not addressed in this book. Nor does this book explain safety and health issues. 

1. Molecular weight and molecular weight distribution

The high average molecular weight and narrow molecular weight distribution in solution polymerised elastomers and the moderate shear forces in the kneader result in a more homogeneous dispersion of the carbon black.

2.Resin additives

Adding resin homogenizers to rubber compounds or blended rubber compounds can improve the dispersion of carbon black in halogenated butyl rubber compounds. Some resin processing AIDS (such as Struktol 40 MS) and rubber asphalt (mineral rubber) can improve the dispersion of carbon black in halogenated butyl rubber. At the same time, it can also improve the compatibility of halogenated butyl rubber with other unsaturated rubbers.

3.Other additives

Fatty amine additives can effectively reduce the dispersion time of ultrafine carbon black with high filling amount in the rubber compound, that is, it can improve the dispersion of carbon black within a certain period of time.

Vinyl acetate wax can shorten the mixing time of highly filled ultrafine carbon blacks, and if the mixing time is appropriately prolonged, it will help the dispersion of ultrafine carbon blacks.

4.Avoid using different carbon blacks together

In order to make carbon black disperse well, we should try to avoid the combination of different carbon blacks (such as N330 and N650), and use a single species of carbon black to fill the rubber.

5.Carbon black

Try to avoid the use of hard granulated carbon blacks, which are less likely to be broken up during the mixing process and do not facilitate better dispersion.Determine the optimum filler level of the carbon black for optimum dispersion.Carbon blacks with a low specific surface area are easier to disperse, as are those with a high degree of structure.Carbon blacks with a high degree of structure and a low specific surface area are easier to disperse and, conversely, carbon blacks with a low degree of structure and a high specific surface area are less easy to disperse.Low structure carbon blacks are the most easily wetted in a blend, but tend to be less easily dispersed.

Super Abrasive Furnace Blacks (SAF), such as N110, are difficult to disperse uniformly.One way to improve dispersion is to have the masterbatch of the carbon black prepared by the elastomer supplier, so that better dispersion can be achieved in a shorter period of time during subsequent compounding.

6.Kneading order

Typically, high structure, large particle size carbon blacks are the easiest to disperse.However, in an EPDM adhesive made with a 120:105 ratio of carbon black to oil (mass ratio), an anomaly occurred in which the low-structure N326 dispersed better than the high-structure N351. Hess explains it this way: if the oil absorption capacity of the carbon black is much greater or much less than the amount of oil filled in the compound, then the high structural, large particle size carbon black disperses best, but if the oil absorption capacity of the carbon black is exactly the same as the amount of oil filled in the compound, then the carbon black absorbs the majority of the oil and reduces the rate of dispersion of the carbon black. One way to correct this phenomenon is to reverse the mixing order, i.e., mix the carbon black before the oil, which will facilitate proper dispersion of the carbon black.

7.Mixed-phase cracking process

For blends like NR/BR, the phase blending method can be adopted to forcibly disperse fine-grained carbon black in the natural gel phase to achieve the best dispersion.

8. The affinity of carbon black for different elastomers during mixing

In blends of different elastomers, due to the incompatibility of the two phases, there will be continuous phases and dispersed phases in the rubber compound. It is very important that the affinity of high-wear-resistant furnace black for different elastomer phases varies. Research has found that all reinforcing carbon blacks have a strong affinity for BR and SBR, while their affinity for CR and NBR weakens. NR follows, and butyl rubber has the poorest affinity. Therefore, in order to achieve a relatively uniform dispersion of carbon black in different phases of the blend, it is necessary to adopt the phase blending method.

In BR/BIIR blend rubber compounds, carbon black is almost entirely dispersed in the BR phase. By adding a compatibilizer (a mixture of aliphatic - naphthenic - aromatic resins), the distribution of carbon black in the BIIR phase of butyl bromide gum can be effectively improved.

9.Oil    

For SBR and BR rubbers, the use of aromatic oils as processing oils is more favourable for the dispersion of carbon blacks in the rubbers.

10.The sequence of adding carbon black and oil during mixing

Normally, for better dispersion of carbon black, the addition of oil and carbon black at the same time should be avoided, and the best sequence is to add carbon black first, then oil.

11.Mixed rubber left overnight

Often, the dispersibility of carbon black can be significantly improved by leaving a section of the mix to cool overnight and then returning it to the mix.

12.Silica

For silica-filled compounds, use highly dispersed silica (e.g. Hi-Sil EZ), which is well dispersed in the compounds.

13. Processing AIDS that help disperse silica

In the case of silica-silane coupling agents used in one-stage blends, hybrid resin-based processing aids such as Struktol XPI343 or fatty acid ester-based processing aids such as Struktol XP 1335 are considered because they are specifically designed to minimize the effect of silane reactions.

14.Dispersion of silica in polar elastomers

Silica disperses better in polar elastomers than in non-polar elastomers, resulting in better reinforcement.

15.In-situ silica generation in rubber

It is difficult to make the dispersion of silica uniform . A possible experimental method is the in situ production of silica in rubber by means of tetraethoxysilane, which involves a hydrolysis and condensation reaction, and which results in a well-dispersed silica.

16.Dispersion of fibers

A 5 parts by weight fibre slurry (e.g. cotton, nylon 6 or polyester) with low molecular weight maleic anhydride polybutadiene (PBDMA) can effectively shorten the blending time of fibres in other adhesives such as EPR, where PBDMA acts as a bulking agent between the fibres and the rubber, facilitating rapid soaking of the fibres in the rubber.    

17.Carbon black masterbatch

Polymer manufacturers offer elastomer masterbatches that are pre-blended with carbon black, which can be used to shorten mixing time and result in a better dispersed rubber.

18.Masterbatch

For better dispersion of aramid fibers, a masterbatch mixed with Kevlar fibers can be used, which is available from fiber manufacturers.

19.Polymer gels

A small amount of "loose" gels (equivalent to the high gel content of natural rubber) are present in the combined polyisoprene rubber, which increases the viscosity of the rubber to a certain extent, but is easily broken during mixing and aids in the dispersion of the filler. However, in the case of "tight" gels, which cannot be broken during mixing, the various properties of the gel are jeopardized.

20.Open mill mixing

 When mixing with an open mill, it is necessary to avoid mixing hard rubber and soft rubber together. Generally, rubber types with similar hardness are more likely to be mixed with each other. Mixing with an open mill can also ensure that the rubber blocks will not be rapidly broken before all the compounding agents are well dispersed. For high-filling rubber compounds, dry fillers should not be added to the open mill quickly. Instead, they should be added gradually, allowing a small portion of the fillers to interact with the rubber compound first. The addition of oil and plasticizers is often after the filler. Of course, this is not set in stone. If the amount of filler oil is large, it is an exception.     

The key to the mixing process in an internal mixer is to maintain the stiffness of the rubber compound. This is because the rubber compound should not be completely crushed and softened until all the compound components are well dispersed. Therefore, oil and plasticizers are often added in the later stage of the mixing process. The feeding sequence for internal mixer mixing is generally as follows: pure rubber and part of the filler; The remaining packing material Oil and plasticizers. However, if the rubber compound will be softened rapidly, the order of adding materials should be reversed.

22.Two-stage mixing

To achieve better dispersion, a two-stage mixing method can be considered. The first stage prepares the masterbatch rubber without the sulfurization system, and the second stage can have a longer mixing time, which is conducive to the dispersion of fillers. This method is particularly suitable for rubber compounds filled with fine particle size carbon black or silica.

23.The cooling system on the internal mixer

Use a refiner with a cooling system to ensure that the mixing temperature does not rise too quickly. If the temperature rises too quickly, on the one hand, shorten the burning time, on the other hand, the viscosity of the rubber drops rapidly, the shear force is reduced, which is not conducive to the uniform dispersion of the filler.Therefore, keep the mixing temperature low, can increase the mixing time, to ensure that the filler is fully dispersed.Assuming a certain mixing volume and a cooling system, the temperature of the rubber can also be adjusted by adjusting the top bolt pressure and rotor speed.

24.Continuous mixing system

If a continuous mixing system is used, the key lies in how to set appropriate conditions to achieve the full dispersion and distribution of the compound components, but this is not very easy to achieve.

Extracted from "1500 Practical Solutions for Improving the Performance of Rubber Compounds"